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Core classes of the flexible query parser framework.

<h2>Flexible Query Parser</h2>

<p>
This package contains the necessary classes to implement a query parser.  
</p>

<p>
A query parser is divided in at least 2 phases, text parsing and query building, and one optional phase called query processing.
</p>

<h3>First Phase: Text Parsing</h3>
<p>
The text parsing phase is performed by a text parser, which implements {@link org.apache.lucene.queryparser.flexible.core.parser.SyntaxParser} interface.
A text parser is responsible to get a query string and convert it to a {@link org.apache.lucene.queryparser.flexible.core.nodes.QueryNode} tree,
which is an object structure that represents the elements defined in the query string.
</p>

<h3>Second (optional) Phase: Query Processing</h3>
<p>
The query processing phase is performed by a query processor, which implements {@link org.apache.lucene.queryparser.flexible.core.processors.QueryNodeProcessor}.
A query processor is responsible to perform any processing on a {@link org.apache.lucene.queryparser.flexible.core.nodes.QueryNode} tree. This phase
is optional and is used only if an extra processing, validation, query expansion, etc needs to be performed in a {@link org.apache.lucene.queryparser.flexible.core.nodes.QueryNode} tree.
The {@link org.apache.lucene.queryparser.flexible.core.nodes.QueryNode} tree can be either be generated by a text parser or programmatically created.
</p>

<h3>Third Phase: Query Building</h3>
<p>
The query building phase is performed by a query builder, which implements {@link org.apache.lucene.queryparser.flexible.core.builders.QueryBuilder}.
A query builder is responsible to convert a {@link org.apache.lucene.queryparser.flexible.core.nodes.QueryNode} tree into an arbitrary object, which
is usually used to be executed against a search index.   
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